The invention relates to lamps adapted for use as headlamps on automobiles, and for other uses where a tungsten incandescent lamp has been conventionally used but where the intensity and color rendition of an arc lamp would also be desirable.
Several lamps in which an arc discharge has been combined with an incandescent filament have been developed. A typical lamp consisted of a mercury arc discharge between tungsten electrodes at the terminals of a tungsten filament. Such a mercury arc lamp has been used particularly in the home as sunlamps for obtaining skin tans indoors.
One great disadvantage of such lamps is that the arc discharge in vapor lamps has a negative resistance characteristic. This means simply that a higher current in the arc reduces the resistance of the arc. Without some sort of stabilization a runaway condition occurs in which the higher current lowers the resistance of the arc which further increases the current until the electrodes themselves are destroyed. To overcome this, the arc lamps normally require an external ballast resistance to keep the current within acceptable levels so as not to destroy the lamp. Because of the energy which must be dissipated in the ballast resistor, the efficiency of the lamp circuit is lowered.